The core of an aircraft wing is a section called a wing box. The wing box is faired into the aircraft fuselage and runs from the root towards the tip of the wing. The wing box provides the central sections of the upper and lower aerofoil surfaces for the wing in addition to attachment points for engines and control surfaces such as ailerons, trim flaps and airbrakes. The aerofoil surfaces of the wing box are provided by panels referred to as wing covers. The wing covers include a number of structural elements called stringers, which run within the wing box structure from the root towards the tip. The stringers are arranged to provide the necessary structural stability and integrity to enable the wing covers to cope with the operational loads on the wing. Stringers also provide important attachment points for other parts of the wing box structure. For example, wing ribs are commonly braced to the stringers by rib cleats fastened between these two structures.
When building aircraft structures, one of many considerations is the balance between weight and strength. In other words, structures need to be as light as possible while providing the necessary strength and structural integrity to perform properly under operational loads. Some types of stringers such as box section stringers or hat section stringers are designed with closed cross-sections. Other types of stringers have open or semi-open cross sections such as J or Y section stringers respectively. These open or semi-open section designs are generally simpler to maintain and repair than closed section designs. The weight efficiency of any stringer shape depends on a balance of the thicknesses and widths of its section shape as well as section shape. Generally, Y and hat section designs are more efficient than I or J section designs. In other words, closed or semi-open section designs are generally more efficient than open section designs.
Repairing both closed, open or semi-open section stringers presents problems. Repairs are carried out when damage from cracking or corrosion is identified in part of a stringer. A repair generally involves fastening one or more plates in the form of doubler plates over the affected area. Closed or semi-open section stringers, such as hat section stringers or Y section stringers, can only be repaired using a relatively large number of blind fasteners, that is a fastener with one end within the closed section. Blind fastenings are difficult to fully inspect for serviceability. Repairing some stringers, such as Y section stringers, requires a complex set of doubler plates designed to conform the surface being repaired. J or I section stringers require less complex doubler plates but are generally less weight efficient than Y and hat section stringers as noted above.